According to the video’s description from YouTube user mlcarriker, one day while his brother was out in the woods he found this deer and they took care of it until it could eat on its own.

You will see the dog and deer wrestle, and a few times the cameraperson tells the dog not to bite Theen, as they call him. Apparently he does fit in with the wild herd, but continues to come back for food and to play with their dog.